Nested loop with value comparison in Ansible - ansible

Question

in the example below, how do I retrieve the grade of students listed in selected_students_list? I need to process the grade of the student. In other programming languages I can use nested loops but I'm a bit confused how to implement this in Ansible
---
- name: Test
hosts: localhost
vars:
- all_students_list:
- {
name: "Charlie",
grade: 9,
}
- {
name: "Alice",
grade: 8,
}
- {
name: "Bob",
grade: 7,
}
- {
name: "Darla",
grade: 10,
}
- {
name: "Edith",
grade: 11,
}
- selected_students_list:
- Alice
- Bob
tasks:
- name: Process selected students
debug:
msg: "<PROCESS THE GRADE OF SELECTED STUDENTS i.e. Alice and Bob>"
with_nested:
- "{{ all_students_list }}"
- "{{ selected_students_list }}"

Answer 1

It's possible either to list selected students item by item. For example
- name: Process selected students
debug:
msg: "PROCESS THE GRADES [{{ item.grade }}]
OF SELECTED STUDENT {{ item.name }}"
loop: "{{ all_students_list|
selectattr('name', 'in', selected_students_list)|
list }}"
gives
msg: PROCESS THE GRADES [8] OF SELECTED STUDENT Alice
msg: PROCESS THE GRADES [7] OF SELECTED STUDENT Bob
Or, to display the whole list at once. For example
- name: Process selected students
debug:
msg: "{{ msg.split('\n')[:-1] }}"
vars:
my_list: "{{ all_students_list|
selectattr('name', 'in', selected_students_list)|
list }}"
msg: |
PROCESS THE GRADES OF SELECTED STUDENTS
{{ my_list|to_yaml }}
gives
msg:
- PROCESS THE GRADES OF SELECTED STUDENTS
- '- {grade: 8, name: Alice}'
- '- {grade: 7, name: Bob}'
If the data is in a dictionary, instead of a list, the listing item by item is very simple. For example
- name: Test
hosts: localhost
vars:
- all_students_dict:
Charlie:
grade: 9
Alice:
grade: 8
Bob:
grade: 7
Darla:
grade: 10
Edith:
grade: 11
- selected_students_list:
- Alice
- Bob
tasks:
- name: Process selected students
debug:
msg: "PROCESS THE GRADES [{{ all_students_dict[item].grade }}]
OF SELECTED STUDENT {{ item }}"
loop: "{{ selected_students_list }}"
gives
msg: PROCESS THE GRADES [8] OF SELECTED STUDENT Alice
msg: PROCESS THE GRADES [7] OF SELECTED STUDENT Bob
It's possible to use filters dict2items and items2dict to create a dictionary of selected students. For example
- name: Process selected students
debug:
msg: "{{ msg.split('\n')[:-1] }}"
vars:
my_dict: "{{ all_students_dict|
dict2items|
selectattr('key', 'in', selected_students_list)|
list|
items2dict }}"
msg: |
PROCESS THE GRADES OF SELECTED STUDENTS
{{ my_dict|to_yaml }}
gives
msg:
- PROCESS THE GRADES OF SELECTED STUDENTS
- 'Alice: {grade: 8}'
- 'Bob: {grade: 7}

Answer 2

This should do
vars:
...
query: "[?name == `{{ item }}`].grade"
tasks:
- name: Process selected students
debug:
msg: "{{ all_students_list | json_query(query) }}"
loop: "{{ selected_students_list }}"
Check json-query-filter for more details.